[indybird]

If nvidia takes the GX2 route again with the GTX 280, then (assuming the GX2 performs equal to two GTX 280s in SLI), it will be approximately equal to the HD4870X2.

Heres my reasoning:
-In SLI the second GTX 280 adds anywhere from 50% to 80% of the performance of a single card (according to most reviews).
-The ATI HD4870X2 is claimed to be 50% to 80% faster than the GTX 280
-Therefore, unless nvidia improves their scaling or ATI's isnt as good as they are currently claiming, these two cards will be equal in performance
-Based on the cost of a 65nm GTX 280 core, the GTX 280 will be very expensive: ~$700.

However, if nvidia chooses to compete with the HD4870X2 using the 55nm (probably overclocked) GTX 280 then I believe that the HD4870X2 will be more powerful.

More reasoning:
-When nvidia lowered the manufacturing process of the 9800GTX to 55nm, overclocked it and called it the 9800GTX+, the average performance gain was 10%, 20% at absolute best (also according to reviews).
-If the ATI HD4870X2 is 50% to 80% faster then the GTX 280 then the HD4870X2 will be app. 35% to 60% faster then the GTX 280 55nm/overclocked.

Sounds to me like the best option for nvidia would be a GTX260GX2 with the cores overclocked to GTX280 speeds and to sell it for about $550-$600.

-Indybird